1. Field of The Invention
The present invention relates to a fixing apparatus provided on an electrophotographic image forming system such as a copying machine, printer or fax machine, and an image forming apparatus using the same.
2. Description of Related Art
The fixing apparatus, used so far in the electrophotographic image forming system such as a copying machine, printer or fax machine, having a heating roller member (heating roller or upper roller) and a pressure roller member (pressure roller or lower roller), is often the fixing apparatus of a heating roller fixing type, wherein the transfer material with unfixed toner image formed thereon is heated as it is held and transported by a heating roller having an elastic layer (rubber roller layer) and maintained at a predetermined temperature and a pressure roller in contact with this heating roller and equipped with an elastic layer.
In the apparatus of this type, however, the heating roller has a large heat capacity, and hence requires a longer heating up time, resulting in a higher temperature inside the elastic layer and a reduced service life of the heating roller.
Further, there has been a growing requirement for energy saving, and development efforts are being made to provide a fixing apparatus that ensures a higher fixing performance without increasing the power consumption. There has been a trend for upgrading the surface gloss of the image by the fixing section in order to meet the requirements for higher quality. To provide a technique for improving the image quality, an image forming apparatus using an oil-free fixing apparatus (where no oil is used) in the fixing process is coming on the market.
A device for separating stacked materials including the transfer materials and documents is disclosed in the Official Gazette of Japanese Patent Tokkaihei 6-48600 containing the description of a stacked material separator capable of forming the separation member by a simple method and transporting the separated materials without the stacked materials being damaged.
However when the separation function of the above-mentioned stacked material separator or document image reader is applied to the above-mentioned fixing apparatus, and a flat fixing nip portion is formed by adjusting the hardness of the elastic layer (rubber roller layer) of the fixing roller having a heating roller member (heating roller or upper roller) and pressure roller member (pressure roller or lower roller), an oil application mechanism becomes necessary if separability of the filled-in image is to be ensured. This has presented a problem from the viewpoint of paper handing and copying costs.
To solve this problem, the fixing nip portion was made in upwardly convex shape (for increasing the distortion of the heating roller member) so as to prevent it from jamming on the heating roller member (upper roller), and to eliminate the need of using an oil application mechanism, thereby upgrading the paper handling performance and reducing the copying costs. In this case, however, when paper feeding performance of high printing percentage was ensured by increasing the degree of the upwardly convex of the upwardly convex fixing nip portion (for increasing the distortion of the heating roller member), a problem was found in the jamming of the transfer material on the pressure roller member (lower roller) when a blank sheet of paper was fed (during feed of a black sheet of paper).
A document image reader for enhancing the degree of freedom in the document feed direction by preventing the documents from jamming is disclosed in the Official Gazette of Japanese Patent Tokkaihei 7-312675.
The Official Gazette of Japanese Patent Tokkaihei 6-230695 discloses one of the known examples of the fixing apparatus, wherein an elastic layer having a hardness of 65 degrees (AskerC) and a wall thickness of 3 mm or more is provided on the outer periphery of the shaft; the durability of the fixing roller is improved with respect to:
a heating roller member (heating roller or upper roller) covered with a releasing layer on the outer periphery, and
a pressure roller member (pressure roller or lower roller) incorporating a heater to heating the heating roller member from the outside while rotating in close contact with the heating roller member; and the time for waiting the fixing operation is reduced according to the type of the transfer material to be fixed and the formation of the transmission image.
However, an oil-free fixing apparatus could not be provided by the prior art flat type fixing nip portion having the same hardness on the upper and lower portions; jamming of the transfer material on the heating roller member (upper roller) was observed. To solve this problem with jamming, the nip portion was formed in an upwardly convex shape, but the transfer material was curled at the nip portion (fixing nip portion), so it was round around the pressure roller member (lower roller). When an image was formed on both faces of the transfer material, the transfer material came in contact with the separation jaw of the lower roller (pressure roller member) to cause a damage to be produced on the image.
In an electrophotographic image forming system, the transfer material such as recording paper with a toner image of a photoconductor transferred thereon is transported to the nip portion of the heating roller and pressure roller member rotating in contact with each other, whereby fixing is carried out. In this case, the toner on the transfer material is offset on the heating roller, the transfer material are jammed on the heating roller or pressure roller, or the surface gloss is changed by the mutual pressing force between both rollers.
One of means for avoiding such offsetting or jamming onto the heating roller is disclosed in the Official Gazette of Japanese Patent Tokkai 2001-42679, where electric charge of the same polarity as that of the toner and opposite to that of the transfer material is applied to two rollers by the electric charge produced when the transfer material is transported by the heating roller and pressure roller, and a greater amount of electric charge is applied to the pressure roller member in such a way that the transfer material is fed closer to the pressure roller member.
One of the means for solving the problem with changing the surface gloss is described in the Official Gazette of Japanese Patent Tokkai 2003-15459, which discloses an adjusting device where the pressing force resulting from the contact between two rollers can be changed.
However, subtle factors causing instability are found out in the method disclosed in the Official Gazette of Japanese Patent Tokkai 2001-42679, where electric charge of the same polarity as that of the toner and opposite to that of the transfer material is applied and a greater a mount of electric charge is applied to the pressure roller member in such a way that the transfer material is fed closer to the pressure roller member. Considerable difficulties are considered to be involved in the method of stabilizing the surface gloss by using an adjusting device where the pressing force resulting from the contact between two rollers is adjusted, as disclosed in the Official Gazette of Japanese Patent Tokkai 2003-15459.
In view of the prior art problems described above, it is an object of the present invention to avoid the problems that may be raised by the jamming of the transfer material onto the heating roller and pressure roller when an upwardly convex shaped fixing nip portion is formed in the fixing section. Another object of the present invention is to prevent jamming and to enhance the transfer material ejection performance from the fixing section.